This invention relates to a method and apparatus for measuring the capacitance of variable capacitors and to methods and apparatus for measuring relative humidity using capacitive sensors whose capacitance varies with relative humidity. This invention also relates to methods and apparatus for measuring other variables by using capacitive sensors whose capacitance is related to the magnitude of the variable to be measured.
Capacitive humidity sensors may, for example, be constructed by laying a first conductive plate area on a silicon chip, then covering that area with a polymer, such as a polyimide, of desired thickness as a dielectric, and then depositing the second plate as a conductive layer over the dielectric. The polyimide forms a thin, water absorbing dielectric film whose dielectric constant varies in proportion to the concentration of the absorbed water so that the capacitance of the combination varies with the humidity of the surrounding region.
Another example of a capacitive sensor is the capacitive pressure transducer. One form these transducers take at the present involves the use of a silicon diaphragm which is bonded between two silicon plates to form a capacitor on each side of the diaphragm. Those capacitors are responsive to the difference in pressure between the two sides of the diaphragm. The dielectric between the plates in these structures is usually silicone oil.
In both the capacitive pressure transducer and the capacitive humidity sensor, it has been found to be desireable to integrate the measuring circuit and the capacitive sensor onto a single monolithic silicon chip, if that is possible. By so doing, the sensor and the other capacitive elements of the circuit can easily be constructed on the substrate at the same time so that they have the same plate dimensions and the same dielectric thickness. This gives all of the capacitors the same characteristics making it possible to incorporate them into measuring circuits without the need for either electrically trimming those circuits or physically trimming the plate dimensions to attempt to match their characteristics. Also, by using a single chip the components of the circuit will be subjected to the same ambient conditions so that temperature and pressure variations will affect the components of the circuit by the same amount. If placing all of the measuring circuit components on a single chip is not possible, then it has been found to be desirable to make the measuring circuit elements as nearly similar as possible and place them as close as possible to the sensor so that they have very similar characteristics to those of the sensor and are subjected to ambient conditions closely approximating those to which the sensor is subjected.
Typically, the prior art relating to the measurement of capacitance requires the use of resistors. It is well known that large accurate resistors require a significant area on a chip. Also, it is desirable to avoid the need to depend on the accuracy of the parameters introduced into a circuit by a resistor. Similarly, it is desirable to avoid variation which can be introduced by a semiconductor device or by a multivibrator. For these reasons improvements can be foreseen if it is only necessary to depend on the parameter values of capacitors and external reference voltages. One such improvement would be minimizing the cost of manufacture. This benefit is evident when one considers the fact that capacitors can be matched during the mask and layout stage of the semiconductor manufacturing procedure, and the fact that the possibility of closely matching those elements makes trimming unecessary even when one must provide finished units which will all have the same span and the same offset so that they can be used interchangeably without the need for calibration.
Switched capacitor circuits are known in the field of A/D converters. Such circuits have used switched capacitors which are effective to change the input of an amplifier circuit in the manner shown in the publication "Intuitive IC CMOS Evolution" by Frederiksen, at pages 103-105. In those circuits, there is shown a sampled data comparator which consists of CMOS analog switches, a string of capacitively-coupled logic inverters for voltage gain, and capacitors, some of which convert from voltage to charge and others of which serve to couple the converters. The particular circuits described, while not useful in measuring capacitance, do show the use of a string of capacitively coupled logic inverters providing amplification for a switched-capacitor circuit, where the capacitors in the circuit are zeroed by shorting out the logic inverters. That approach is used to provide the amplification and the setting-up of the capacitors in one form of the switched capacitor circuit of the present invention.
It is an object of this invention to provide an improved capacitance measuring circuit and, more particularly, one which will measure the capacitance of a capacitive sensor by using only capacitors and other circuit components which can be easily integrated onto a small monolithic silicon chip so as to avoid the need for either physically trimming the components or electrically trimming the associated measuring circuit for calibration purposes.
It is a further object of this invention to provide a measuring circuit for measuring the capacitance of a capacitive humidity sensor so that a minimum of trimming is needed even though it is not possible to integrate all of the capacitors of the measuring circuit onto the same silicon chip.
In addition, it is an object of this invention to provide a measuring circuit for measuring the capacitance of a capacitive humidity sensor in a manner which will make the measurement immune to drift with changes in temperature or humidity.